Zack Rusin Talks About Gallium3D's TGSI IR

Lately there's been a lot of talk about Gallium3D's IR known as TGSI, or Tokenized Gallium Shader Instructions, and attempts by some to replace this intermediate representation. Efforts toward improving TGSI are not particularly new, but it's been going on for a while and then just earlier this month a new shader and compiler stack was proposed by LunarG. As part of the LunarGLASS proposal, the LLVM IR would be used as a replacement to TGSI.

Zack Rusin of VMware (formerly with Tungsten Graphics) has just written a blog post about Gallium3D's Tokenized Gallium Shader Instructions for those interested in more information regarding the technical side of the IR and its current shortcomings and hurdles being experienced by these open-source GPU driver developers.

Zack's blog post about the IR can be read on his Blogger page. As some takeaway comments for those not interested in all of the technical details but just an overview of the situation, "[Tungsten Graphics'] lofty goal of making the IR so much easier and quicker to adopt took a pretty severe beating. It's especially disappointing since if you look at some of the documentation e.g. for AMD Intermediate Language you'll notice that this stuff is essentially Direct3D assembly which is essentially TGSI (and most of the parts that are in AMD IL and not in TGSI are parts that will be added to TGSI) . So they have this code. In the case of AMD it's even sadder because the crucial code that we need for OpenCL right now is OpenCL C -> TGSI LLVM backend which AMD already does for their IL. Some poor schmuck will have to sit down and write more/less the same code."

The most important paragraph in Zack's blog post is perhaps, "So we're left with Free Software developers who don't have access to the Direct3D functional spec and who are being confused by the IR which is unlike anything they've seen (pre-declared registers, typeless...) which on top of it is not easily transformable. TGSI is very readable, simple and pretty easy to debug though so it's not all negative. It's also great if you never have to optimize or transform its structure which unfortunately is rather rare. If we abandon the hope of having the code from Windows drivers injected in the GNU/Linux drivers it becomes pretty clear that we could do better than TGSI. Personally I just abhor the idea of rolling out our own IR. IR in the true sense of that word. Crazy as it may sound I'd really like my compiler stuff to be written by compiler experts. It's the main reason why I really like the idea of using LLVM IR as our IR."